/* ----------------------------------------------------------------------
* Copyright (C) 2010 ARM Limited. All rights reserved.
*
* $Date:        15. February 2012
* $Revision: 	V1.1.0
*
* Project: 	    CMSIS DSP Library
* Title:	    arm_fir_decimate_q31.c
*
* Description:	Q31 FIR Decimator.
*
* Target Processor: Cortex-M4/Cortex-M3/Cortex-M0
*
* Version 1.1.0 2012/02/15
*    Updated with more optimizations, bug fixes and minor API changes.
*
* Version 1.0.10 2011/7/15
*    Big Endian support added and Merged M0 and M3/M4 Source code.
*
* Version 1.0.3 2010/11/29
*    Re-organized the CMSIS folders and updated documentation.
*
* Version 1.0.2 2010/11/11
*    Documentation updated.
*
* Version 1.0.1 2010/10/05
*    Production release and review comments incorporated.
*
* Version 1.0.0 2010/09/20
*    Production release and review comments incorporated
*
* Version 0.0.7  2010/06/10
*    Misra-C changes done
* -------------------------------------------------------------------- */

#include "arm_math.h"

/**
 * @ingroup groupFilters
 */

/**
 * @addtogroup FIR_decimate
 * @{
 */

/**
 * @brief Processing function for the Q31 FIR decimator.
 * @param[in] *S points to an instance of the Q31 FIR decimator structure.
 * @param[in] *pSrc points to the block of input data.
 * @param[out] *pDst points to the block of output data
 * @param[in] blockSize number of input samples to process per call.
 * @return none
 *
 * <b>Scaling and Overflow Behavior:</b>
 * \par
 * The function is implemented using an internal 64-bit accumulator.
 * The accumulator has a 2.62 format and maintains full precision of the intermediate multiplication results but provides only a single guard bit.
 * Thus, if the accumulator result overflows it wraps around rather than clip.
 * In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits (where log2 is read as log to the base 2).
 * After all multiply-accumulates are performed, the 2.62 accumulator is truncated to 1.32 format and then saturated to 1.31 format.
 *
 * \par
 * Refer to the function <code>arm_fir_decimate_fast_q31()</code> for a faster but less precise implementation of this function for Cortex-M3 and Cortex-M4.
 */

void arm_fir_decimate_q31(
    const arm_fir_decimate_instance_q31* S,
    q31_t* pSrc,
    q31_t* pDst,
    uint32_t blockSize)
{
	q31_t* pState = S->pState;                     /* State pointer */
	q31_t* pCoeffs = S->pCoeffs;                   /* Coefficient pointer */
	q31_t* pStateCurnt;                            /* Points to the current sample of the state */
	q31_t x0, c0;                                  /* Temporary variables to hold state and coefficient values */
	q31_t* px;                                     /* Temporary pointers for state buffer */
	q31_t* pb;                                     /* Temporary pointers for coefficient buffer */
	q63_t sum0;                                    /* Accumulator */
	uint32_t numTaps = S->numTaps;                 /* Number of taps */
	uint32_t i, tapCnt, blkCnt, outBlockSize = blockSize / S->M;  /* Loop counters */


#ifndef ARM_MATH_CM0

	/* Run the below code for Cortex-M4 and Cortex-M3 */

	/* S->pState buffer contains previous frame (numTaps - 1) samples */
	/* pStateCurnt points to the location where the new input data should be written */
	pStateCurnt = S->pState + (numTaps - 1u);

	/* Total number of output samples to be computed */
	blkCnt = outBlockSize;

	while(blkCnt > 0u) {
		/* Copy decimation factor number of new input samples into the state buffer */
		i = S->M;

		do {
			*pStateCurnt++ = *pSrc++;

		} while(--i);

		/* Set accumulator to zero */
		sum0 = 0;

		/* Initialize state pointer */
		px = pState;

		/* Initialize coeff pointer */
		pb = pCoeffs;

		/* Loop unrolling.  Process 4 taps at a time. */
		tapCnt = numTaps >> 2;

		/* Loop over the number of taps.  Unroll by a factor of 4.
		 ** Repeat until we've computed numTaps-4 coefficients. */
		while(tapCnt > 0u) {
			/* Read the b[numTaps-1] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-1] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 += (q63_t) x0 * c0;

			/* Read the b[numTaps-2] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-2] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 += (q63_t) x0 * c0;

			/* Read the b[numTaps-3] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-3] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 += (q63_t) x0 * c0;

			/* Read the b[numTaps-4] coefficient */
			c0 = *(pb++);

			/* Read x[n-numTaps-4] sample */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 += (q63_t) x0 * c0;

			/* Decrement the loop counter */
			tapCnt--;
		}

		/* If the filter length is not a multiple of 4, compute the remaining filter taps */
		tapCnt = numTaps % 0x4u;

		while(tapCnt > 0u) {
			/* Read coefficients */
			c0 = *(pb++);

			/* Fetch 1 state variable */
			x0 = *(px++);

			/* Perform the multiply-accumulate */
			sum0 += (q63_t) x0 * c0;

			/* Decrement the loop counter */
			tapCnt--;
		}

		/* Advance the state pointer by the decimation factor
		 * to process the next group of decimation factor number samples */
		pState = pState + S->M;

		/* The result is in the accumulator, store in the destination buffer. */
		*pDst++ = (q31_t)(sum0 >> 31);

		/* Decrement the loop counter */
		blkCnt--;
	}

	/* Processing is complete.
	 ** Now copy the last numTaps - 1 samples to the satrt of the state buffer.
	 ** This prepares the state buffer for the next function call. */

	/* Points to the start of the state buffer */
	pStateCurnt = S->pState;

	i = (numTaps - 1u) >> 2u;

	/* copy data */
	while(i > 0u) {
		*pStateCurnt++ = *pState++;
		*pStateCurnt++ = *pState++;
		*pStateCurnt++ = *pState++;
		*pStateCurnt++ = *pState++;

		/* Decrement the loop counter */
		i--;
	}

	i = (numTaps - 1u) % 0x04u;

	/* copy data */
	while(i > 0u) {
		*pStateCurnt++ = *pState++;

		/* Decrement the loop counter */
		i--;
	}

#else

	/* Run the below code for Cortex-M0 */

	/* S->pState buffer contains previous frame (numTaps - 1) samples */
	/* pStateCurnt points to the location where the new input data should be written */
	pStateCurnt = S->pState + (numTaps - 1u);

	/* Total number of output samples to be computed */
	blkCnt = outBlockSize;

	while(blkCnt > 0u) {
		/* Copy decimation factor number of new input samples into the state buffer */
		i = S->M;

		do {
			*pStateCurnt++ = *pSrc++;

		} while(--i);

		/* Set accumulator to zero */
		sum0 = 0;

		/* Initialize state pointer */
		px = pState;

		/* Initialize coeff pointer */
		pb = pCoeffs;

		tapCnt = numTaps;

		while(tapCnt > 0u) {
			/* Read coefficients */
			c0 = *pb++;

			/* Fetch 1 state variable */
			x0 = *px++;

			/* Perform the multiply-accumulate */
			sum0 += (q63_t) x0 * c0;

			/* Decrement the loop counter */
			tapCnt--;
		}

		/* Advance the state pointer by the decimation factor
		 * to process the next group of decimation factor number samples */
		pState = pState + S->M;

		/* The result is in the accumulator, store in the destination buffer. */
		*pDst++ = (q31_t)(sum0 >> 31);

		/* Decrement the loop counter */
		blkCnt--;
	}

	/* Processing is complete.
	 ** Now copy the last numTaps - 1 samples to the start of the state buffer.
	 ** This prepares the state buffer for the next function call. */

	/* Points to the start of the state buffer */
	pStateCurnt = S->pState;

	i = numTaps - 1u;

	/* copy data */
	while(i > 0u) {
		*pStateCurnt++ = *pState++;

		/* Decrement the loop counter */
		i--;
	}

#endif /*   #ifndef ARM_MATH_CM0 */

}

/**
 * @} end of FIR_decimate group
 */
